This invention relates to a method of forming threshold switching devices which exhibit negative differential resistance (NDR) and to the devices formed thereby. The method comprises depositing a silicon dioxide film derived from hydrogen silsesquioxane resin between at least two electrodes and applying a voltage above a threshold voltage across the electrodes. Such devices are useful as variable resistors and variable capacitors.
Numerous devices which exhibit threshold switching are known in the art. For example, Ovshinsky, in U.S. Pat. No. 3,271,591, issued Sep. 6, 1966, describes such devices in which semiconductor materials, such as crystalline or amorphous tellurides, selenides, sulfides or oxides of substantially any metal, are deposited between electrodes. The semiconductors and methods specifically set forth in this reference, however, are not the same as those claimed herein. As is well known in the art, current-voltage or IV curves are graphical representations of plots of measured values of the current I through a particular material or a device, as a function of the applied voltage V. In many instances, it is more convenient to illustrate current in terms of current density "j". Thus, current density is expressed as j=I/A in which "A" is the surface area of a device expressed in square centimeters, As such, the jV curves in this reference differ from those of the present application.
Threshold switching with negative differential resistance is also known in various metal oxide thin films. For instance, Bullot et al., Physica Status Solidi (a) 71, K1 (1982), describe threshold switching in vanadium oxide layers deposited from gels; Ansari et al., Journal of Physics 20 (1987) 1063-1066 describe threshold switching in titanium oxide films formed by thermally oxidizing a titanium metal layer; Ramesham et el., NASA Tach Briefs, December 1989, p. 28, describe the switching in manganese oxide films; and Morgan et al., Thin Solid Films, 15 (1973) 123-131, describe switching and negative differential resistance in aluminum oxide films. The materials and characteristics described in these references, however, differ from those described herein.
The switching and negative differential resistance characteristics of silicon oxide films have likewise been described. For instance, Simmons, Handbook of Thin Film Technology, Chapter 14 (1970), Pages 14-38 to 14-43, describes electronic conduction through thin insulating films, including silicon oxide, as well as their negative resistance and memory characteristics; Al-Ismail et al., Journal of Material Science, 20 (1985) 2186-2192, describe switching and negative resistance in a copper-silicon oxide-copper system; Morgan et al., Thin Solid Films, 20 (1974) S7-S9, describe threshold switching and memory in silicon oxide films; Boelle et al., Applied Surface Science 46 (1990) 200-205, describe the current-voltage characteristics of silica films derived from sol-gel low temperature methods; and Klein, Journal of Applied Physics, Volume 40, Number 7, June (1969) 2728-2740, describe the electrical breakdown of silicon oxide films. As with the prior metal oxide references, however, these too do not describe the methods and characteristics described herein.
Resistors produced form ceramic oxides are also known in the art. For instance, Eijnthoven in U.S. Pat. No. 4,052,340 issued Oct. 4, 1977, and Nagano in U.S. Pat. No. 3,953,375 issued Apr. 27, 1966, describe resistors derived primarily from zinc oxide and titanium oxide, respectively. Such materials and the resultant properties, however, differ from those claimed herein.
Thin film silica coatings derived from hydrogen silsesquioxane resin are also known in the art. For instance, Haluska in U.S. Pat. No. 4,756,977 issued Jul. 12, 1988 describes forming such films by diluting hydrogen silsesquioxane resin in a solvent, applying the solution to a substrate, drying the solvent, and heating. Such coatings are taught therein to provide protection and electrical insulation.
The present inventors have now found that switching devices with desirable features can be formed by depositing a thin, hydrogen silsesquioxane derived silicon dioxide film between at least 2 electrodes and applying a voltage above a threshold voltage across the electrodes. These devices are useful as variable resistors and variable capacitors.
While silicon dioxide capacitors are described in the patent literature, such capacitors do not possess the same properties as the capacitors of the present invention. This is for the reason that the silicon dioxide films of the prior art are manufactured by processes which differ significantly from the process of the present invention. For example, U.S. Pat. No. 3,149,398 issued Sep. 22, 1964, discloses a silicon dioxide capacitor in which the silicon dioxide layer is formed by the decomposition of an alkyltrialkoxysilane such as ethyltriethoxysilane. The U.S. Pat. No. '398 patent includes an alternate method of producing the silicon dioxide layer by dissociation of silicon tetrachloride to a silicon deposit which is converted to silicon dioxide by oxidation. Further, the U.S. Pat. No. '398 patent notes that the layer may be formed by either the direct deposition of silicon dioxide by evaporation, or the evaporation of silicon monoxide onto a substrate followed by oxidation of the deposit to silicon dioxide. However, there is nothing in the U.S. Pat. No. '398 patent to indicate that the silicon dioxide layers produced therein possess the unique characteristics of the silicon dioxide capacitor films produced by the method employed in accordance with the teaching of the present invention.